This invention relates to the field of holography and, more specifically, to a process for making phase holograms in photographic material.
Production of phase holograms has been carried out through a number of different photosensitive processes. Two broad categories of processes related to this invention are bleached silver halide processes and dichromated gelatin processes.
In a bleached silver halide process, silver halide photographic material is exposed in a holographic optical system, developed and fixed afterwhich it is treated with a bleach which converts the silver metal in the exposed areas of the material to a silver salt. Bromine vapor for example is a known bleaching agent and, when applied to the emulsion, a phase volume hologram is formed consisting of silver bromide in a gelatin matrix. The difference in refractive indexes between the silver bromide or other silver salt and the gelatin produces the diffraction that is the basis of the hologram.
The primary difficulties with the bleached silver halide processes are the production of light scatter or noise by the silver halide crystallites in the emulsion and print-out darkening in which the silver halide is reduced to silver metal over time by actinic light.
U.S. Pat. No. 3,755,111 to Norman is both an example of a bleached silver halide process and of a solution proposed for dealing with the print-out darkening problem. In Norman after a silver halide is formed in the bleaching step, the hologram is treated with a composition which desensitizes the silver halide. Another U.S. Pat. No. 3,891,436 to Nishida, is concerned with the bleached silver halide process and proposes the conversion of the silver salt formed in the bleaching step to a compound of another metal to prevent the print-out darkening phenomenon.
A variation of the bleached silver halide process which produces holograms having low noise levels involves the making of a reverse bleached silver halide hologram. In this type process the hologram is developed but not fixed while the metallic silver is bleached to a soluble silver salt which is removed. This reversal bleaching process is described by Lamberts and Kurtz in an article appearing in Applied Optics, Volume 10, No. 6, June 1971, pages 1342-1347, and in U.S. Pat. No. 3,816,130 to Eggers, which further suggests desensitizing of the unfixed residual silver halide left after this bleaching step.
In the Lamberts and Kurtz process a tanning developer of the pyrocatechol type is used which promotes tanning of the gelatin in those regions where the metallic silver is developed out. Tanning is also promoted by the bleach used by Lamberts and Kurtz, a bleach whose principal ingredients are potassium dichromate and sulfuric acid. In Eggers, an improved hologram is produced by using a non-tanning developer although a potassium dichromate and sulfuric acid tanning bleach is used as in Lamberts and Kurtz.
In the reverse bleached silver halide process as with the basic bleached silver halide process the difference in the refractive indexes between the silver halide and the gelatin produces the diffraction which is the basis of the hologram. Also, with these bleached silver halide processes, whether involving direct or reverse bleaching, a photosensitive ingredient is left requiring desensitizing treatment to avoid print-out darkening.
The second broad category of processes related to this invention is the hardened dichromated gelatin type process. While this type process produces a hologram with minimal light scatter or noise and no print-out darkening, the process does not have the light sensitivity or spectral response of the bleached silver halide processes and, in addition, the sensitized dichromated gelatin does not have storage stability and must be used shortly after preparation for best results. To explain, the hardened dichromated gelatin plates are prepared usually from fixed silver halide emulsions. The plates are sensitized by soaking in an ammonium dichromate solution, dried, and then exposed. Following exposure the plates are washed in running water to remove the remaining dichromate sensitizer and dehydrated in isopropanol baths. The phase hologram formed consists of crosslinked or tanned gelatin in a gelatin matrix. The difference in refractive indexes between the tanned and the untanned gelatin produces the diffraction that is the basis for the hologram. The absence of print-out darkening is due to the absence of any light sensitive silver salt in the final product. Thus, no densensitizing treatment is required.
The important feature, a light stable hologram, requiring no desensitizing, which is the result of a product whose diffraction is due solely to the presence of a tanned gelatin in a gelatin matrix has not lead to the use of the tanning effect in the bleached silver halide type process. This despite the fact that in bleached silver halide processes such as those taught by Eggers and Lamberts and Kurtz, tanning bleaches and tanning developers have been used.